In a data transmission system, signals are carried between end terminals over some medium, such as a hard wire line, fiber optic link, radiated electromagnetic wave, etc. For high density usage, the transmission medium carries a plurality of multiplexed data channels. This is particularly common in telecommunication systems, FIG. 1, where the end terminals include multiplexing and demultiplexing equipment and each path therebetween carries a plurality of time division multiplexed digital channels, typically twenty-four in North America and thirty in Europe.
It may be desirable to listen to and/or substitute channels on the transmission line between the end terminals. This would enable an operator at an intermediate location to listen and/or talk to the ends. Equipment providing this capability is known, and referred to as channel banks in a drop and insert configuration, FIG. 2. These banks enable a channel to be dropped from the line, i.e. listened to. The banks also enable a channel to be inserted on the line to substitute for an existing data channel in that time slot.
The standard approach for dropping and inserting channels between end terminals is to use two completely equipped channel banks cross-connected at the voice frequency (VF) interface. In FIG. 2, each of the end terminals is a channel bank having a VF interface to the various telephone subscribers. Drop and insert operation is performed by an additional pair of channel banks in the middle, which are equipped for each channel and convert the digital transmission data on the line to VF analog signals for each channel. There are thus twenty-four VF analog signals passing between the central pair of channel banks, any of which signals may be dropped or inserted.
A drawback of the above approach is the high cost of two completely equipped channel banks, and the wasteful inefficiency thereof when only a small number of channels are dropped and inserted. For example, to drop and insert only one channel would require that all channels be equipped.
Another drawback of the above approach is the degradation of the signal caused by the digital to analog to digital conversion performed by the channel bank pair. This is particularly objectionable when it is desired to use a number of such pairs between the end terminals. Signal degradation limits the number of such pairs and hence the total number of locations for dropping and inserting channels.